Industrial processing plants use control valves in a wide variety of applications from controlling product flow in a food processing plant to maintaining fluid levels in large tank farms. Control valves, which are typically automated, are used to manage the product flow by functioning like a variable orifice or passage. By moving an internal valve component, the valve plug, the amount of product passing through valve body can be accurately controlled. The control valve is typically automated using an actuator and a remotely operated instrument which communicates between a process control computer and the actuator to command flow changes within the valve to achieve the plant operators' desired control strategy. Position sensors play a critical role in maintaining accurate process control.
When the process control computer issues a command to modify flow, the remotely operated instrument must read the present valve position and apply appropriate corrective action through the actuator. A typical actuator is driven by a pressurized air source, which is controlled by the remotely operated instrument. For example, in a spring and diaphragm actuator used on a sliding stem valve, variations in air pressure applied to a large diaphragm cause movement or displacement of the diaphragm. Attached to the diaphragm is an actuator stem, which in turn is connected to the valve plug. By changing air pressure to the diaphragm, the remotely operated instrument can directly position the valve plug and therefore control flow through the control valve. In order to properly control flow, the instrument must always know where the valve plug is and to where it must move in response to the new command. This is accomplished by attaching a position sensor between the remotely operated instrument and the actuator stem. The output of the position sensor may be directly connected to the remotely operated instrument to provide stem position feedback for precise valve control.
Traditional position sensors, such as potentiometers or other electro-mechanical limit switches, require dynamic or moving mechanical linkages to couple movement or displacement into the sensor. Such electro-mechanical limit switches are mounted on the actuator, and are tripped by a moving element when that element is located at mid-stroke, or at either end of the travel of the valve plug. The signals from the limit switch (or switches) are used to operate relays, solenoid valves, or to trigger alarms. In order to avoid damage to the control element, such as in high thrust valve applications, the limit switches can be placed in locations such that movement of the valve stem does not exceed its desired travel length.
In applications where mechanical vibrations caused by turbulent flow exist, system errors or instabilities can reduce the position sensor's reliability by causing millions of operational cycles to accumulate in a very brief time period. The mechanical linkages also have contact or wear points. During rugged service conditions, instabilities can literally “saw apart” the mechanical linkages at the wear points thereby disconnecting the valve stem from the remotely operated instrument. Catastrophic failures of this type destroy valve control and must be avoided. To improve sensor reliability, sensor designs have migrated to non-contacting position detection methods.
One type of non-contacting sensor design is a magnetic position sensor. Magnetic position sensors detect displacement between two objects by attaching a magnetic flux source, typically a magnet, to the first object and a sensor, such as a Hall Effect sensor to the second object. The magnetic flux source presents a magnetic field that is detected by the sensor. Any movement by one or both objects producing relative displacement presents a different portion of the magnetic field to the sensor, thereby changing the output of the sensor. This output can be directly related to the relative displacement between the actuator and the valve stem.
Non-contact position sensors are very adaptable and can measure numerous forms of displacement. However, current non-contacting position sensors are often limited by the method of attaching them to the moving elements. There are numerous commercial examples of position or feedback sensors in remotely operated instruments that still use “contacting” dynamic linkages to couple displacement. One such configuration uses a conventional worm-gear apparatus to directly couple rotary motion to a-non-contacting magneto-resistive element. Although the magneto-resistive element can be classified as a non-contacting sensor, the motion is actually transduced through a “contacting” apparatus and will suffer from decreased reliability just like traditional linkage-based potentiometers.
Additionally, other non-contact position sensors suffer from the inability to reconfigure the magnet flux source to provide a predefined output for various types of displacement measurement (e.g. rectilinear and rotary). Examples of these types of position sensors are found in Riggs et al. U.S. Pat. No. 5,359,288, Wolf et al. U.S. Pat. No. 5,497,081, and Takaishi et al. U.S. Pat. No. 5,570,015.
Additional shortcomings of existing non-contact position sensors include the need for at least two such limit switches to detect opposite ends of travel of the valve plug, the difficulty of implementing such limit switches, and concern for their reliability. The manner in which these and other shortcomings of existing proximity sensors are overcome will be explained in the following Summary and Detailed Description of the Preferred Embodiments.